In the hot-dip coating of metal strip, in particular steel strip, the strip to be coated is fed through the molten metal by means of a deflection roller arranged in the melt bath. In addition, the portion of the strip emerging from the melt bath is usually guided by guide rollers (so-called “stabirollers”, bottom rollers, stabilizing rollers or passline rollers) likewise arranged in the melt bath, in order to ensure a substantially vertical and oscillation-free passage of the strip through a gap defined by wiper nozzles. The wiper nozzles serve to wipe excess coating material from the strip and to regulate the layer thickness of the coating material adhering to the strip.
The defection rollers and guide rollers used in hot-dip coating units in the state of the art are designed as solid rollers, open shell rollers or closed hollow rollers.
Closed hollow rollers are still used today, but can only be operated with costly ventilation or venting systems, since with the closed roller cavity the high temperatures in the melt bath can lead to very high internal temperatures and even explosive failure of the roller, especially in the event of unwanted fluid inclusions in the roller cavity.
Since the deflection and guide rollers in the melt bath are not normally provided with their own drive, but are turned by the movement of the strip wrapping around them, the use of solid rollers is problematical, particularly in the hot-dip coating of thin strips. The high weight of solid rollers makes it harder for the strip to turn the rollers, particularly in the case of thin strips, which can lead to relative movements (slip) between the strip and the roller. This can cause scratches and other coating defects, which reduce the coating quality.
Shell rollers usually comprise a hollow cylindrical shell tube, which at both ends is connected to the bearing journals by spoke-like webs or openings, for example end plates having bored holes. Melt bath rollers in the form of shell rollers have the advantage that they are easier to turn than heavy solid rollers, and in contrast to the closed hollow rollers do not have an enclosed cavity, in which a critical internal pressure can occur. The end-face openings create a pumping effect, however, as the shell rollers rotate, giving rise to a turbulent material flow of molten metal in the area of the spoke-like webs and openings of the end plates, which leads to a very considerable material abrasion (wear) in the area of the webs and end plates. For this reason the service life of shell rollers used in hot-dip coating units is relatively low.
Common to the said design types of deflection and guide rollers is the fact that their pivot bearings usually take the form of plain bearings, wherein the bearing journals of the rollers (and those of the outer bearings receiving the bearing journals) are subject to severe wear influences. The high degree of wear of the bearing journals, typically made from steel, is mainly caused by the high bearing forces and the aggressive nature of the molten metal.